1. Field of the Invention
This invention relates to an exhaust gas-purifying system for internal combustion engines, and more particularly to an exhaust gas-purifying system of this kind which has an exhaust gas-purifying device with an absorbent arranged in the exhaust system of the engine, for absorbing nitrogen oxides in exhaust gases emitted from the engine.
2. Prior Art
When an internal combustion engine operates in a condition where the air-fuel ratio of a mixture supplied to the engine is set to a leaner value than a stoichiometric air-fuel ratio to carry out so-called lean-burn control, it is likely that an increased amount of nitrogen oxides (hereinafter referred to as "NOx") is emitted from the engine. To overcome this disadvantage, conventionally an exhaust gas-purifying device with a NOx absorbent is arranged in the exhaust system of the engine, for absorbing NOx in exhaust gases emitted from the engine, to thereby carry out purification of exhaust gases emitted from the engine. The NOx absorbent has such a characteristic that it absorbs NOx when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio and accordingly the concentration of oxygen present in exhaust gases is relatively high, i.e. the amount of NOx is large (hereinafter referred to as "the exhaust gas lean state"), whereas it desorbs the absorbed NOx when the air-fuel ratio is richer than the stoichiometric air-fuel ratio and accordingly the concentration of oxygen present in exhaust gases is low, i.e. the amount of HC and CO is large (hereinafter referred to as "the exhaust gas rich state"). Therefore, in the exhaust gas rich state, the exhaust gas-purifying device with the NOx absorbent functions to reduce NOx desorbed from the NOx absorbent to a nitrogen gas by reaction with HC and CO, which is emitted into the air, and oxidize HC and CO into steam and carbon dioxide, which are also emitted into the air.
The NOx absorbent, however, has a limited capacity for absorbing NOx, and therefore the lean-burn control cannot be continued over a long time period. To cope with this inconvenience, there is conventionally known a method, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 7-139340, which temporarily enriches the air-fuel ratio in order to desorb NOx which has been absorbed by the NOx absorbent, for reduction of the thus desorbed NOx. In the present specification, this temporary enrichment of the air-fuel ratio for desorbing NOx will be referred to as "reduction enrichment".
Japanese Laid-Open Patent Publication (Kokai) No. 7-139340 referred to above also discloses a method of estimating an amount of NOx absorbed by the NOx absorbent to suitably carry out the reduction enrichment. According to the method, an NOx amount-estimating counter is provided for estimating an amount of NOx absorbed by the NOx absorbent, and the count value of the NOx amount-estimating counter is incremented during execution of the lean-burn control, while it is decremented during execution of the reduction enrichment or air-fuel ratio feedback control with the desired air-fuel ratio set to a stoichiometric air-fuel ratio. More specifically, during execution of the lean-burn control, a predetermined addend set according to operating conditions of the engine is added to the count value of the NOx amount-estimating counter at predetermined time intervals, while during execution of the reduction enrichment or the air-fuel ratio feedback control with the desired air-fuel ratio set to the stoichiometric air-fuel ratio, a predetermined subtrahend is subtracted from the count value of the NOx amount-estimating counter at predetermined time intervals. The subtrahend is set according to the temperature of the NOx absorbent and an excess of a fuel amount supplied to the engine during the reduction enrichment in excess of a required fuel amount for obtaining the stoichiometric air-fuel ratio.
The conventional absorbed NOx amount-estimating method, however, does not contemplate the actual pressure within the cylinder and the actual temperature within the cylinder in estimating the absorbed NOx amount, which results in a large error in the estimated absorbed NOx amount. Consequently, timing of starting the reduction enrichment can become earlier or later than the desired timing. As a result, the fuel amount for enriching the air-fuel ratio can be increased to degrade fuel economy is degraded, or the timing of starting the reduction enrichment can be delayed to increase the emission amount of NOx.